drivers/iio/frequency/ad9122: Enable die temperature measurements

Writing in_temp0_input with the device temperature in milli degrees Celsius
triggers the calibration. Reading in_temp0_input without previous calibration
returns -EINVAL. Reading in_temp0_input after calibration returns the
die temperature in milli degrees Celsius.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>